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If you’ve ever ended up at a greasy diner ravenous at 4 in the morning, this will come as no surprise: new studies find that a sleepy brain is more drawn to junk food than a well-rested one, and that it’s also less equipped to fend off those unhealthy cravings.

A growing body of research has associated sleepiness with overeating, which can in turn lead to weight gain and increased risks of diabetes. Part of the problem is that lack of adequate sleep throws the body’s circadian rhythm and metabolic systems out of whack. Now two small new studies, both presented at the SLEEP 2012 conference in Boston, shed further light on the sleep-weight connection: the first study by researchers at Columbia University finds that sleep deprivation affects the way the brain responds to reward — namely junk food. The second study by researchers at the University of California, Berkeley, finds that sleepiness also appears to hinder the brain’s higher-order functions — like complex decision-making — making sleepy people less able to forgo that middle-of-the-night burger and fries.

In the Columbia study, researchers gave 25 healthy men and women brain scans after five consecutive nights of either adequate sleep (up to 9 hours each night) or sleep deprivation (4 hours a night). The participants had their brains scanned with functional magnetic resonance imaging (fMRI) while they looked at photos of healthy or unhealthy foods.

Researchers found that in brain networks associated with reward were more active in the sleepy people — more so when they looked at pictures of junky foods like pizza and cake than when they were shown healthy foods like fruits, veggies or oatmeal.

“The results suggest that, under restricted sleep, individuals will find unhealthy foods highly salient and rewarding, which may lead to greater consumption of those foods,” said lead study author Dr. Marie-Pierre St-Onge in a statement. “Indeed, food intake data from this same study showed that participants ate more overall and consumed more fat after a period of sleep restriction compared to regular sleep. The brain imaging data provided the neurocognitive basis for those results.”

The researchers theorize that when people don’t get enough sleep, their bodies may naturally seek high-calorie foods to help them make it through the day.

In the second study, which was similar to the Columbia study, researchers at Berkeley’s Sleep and Neuroimaging Laboratory used fMRI to gauge brain activity in 23 healthy adults while they rated how much they wanted various food items shown to them during the scan. Participants were scanned twice, once after a normal night’s sleep and also after a night of total sleep deprivation — zero sleep.

Contrary to the Columbia study, the researchers didn’t find that the participants’ reward centers responded more strongly when they were sleepy. Rather, they found that when participants were sleep-deprived, their brain activity in the frontal lobe — the region that helps control behavior and guides complex decision-making — was impaired. That suggests that sleepiness hinders the brain from weighing the pros and cons and making smart decisions. As far as food goes, that means going for tasty junk food instead of healthy fare.

“We did not find significant differences following sleep deprivation in brain areas traditionally associated with basic reward reactivity,” said lead study author and graduate student Stephanie Greer in a statement. “Instead, it seems to be about the regions higher up in the brain, specifically within the frontal lobe, failing to integrate all the different signals that help us normally make wise choices about what we should eat.”

The differences in findings between the two studies could be chalked up to the extent of the sleep deprivation, Dr. Michelle Miller, a sleep researcher at the University of Warwick Medical School in the U.K., told CNN. The sleepy participants in the Columbia study at least got to sleep four hours a night, while those in the Berkeley experiment had no sleep whatsoever. Both impaired decision-making and increased pleasure-seeking may play a role in food cravings, said Miller, but higher-order impairments may become more pronounced with lack of sleep.